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Old 14th March 2007, 12:37 PM   #1
PxPert is offline PxPert  Italy
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Default cmoy amp - Bad sound

Hello to all! First of all great forum, it's my first post

I've just bought a cmoy amp from ebay, here is the link -> http://cgi.ebay.it/ws/eBayISAPI.dll?...5178&rd=1&rd=1

It seems to work, but from middle to high volume the sound, especially the low frequences, distorts. I don't like this because the output signal from my audio card without the amp is higher (and cleaner) than with that...

Can I make some changes to solve this problem?

Here are some photos of the cmoy
http://img137.imageshack.us/img137/3869/img00211mm8.jpg
http://img154.imageshack.us/img154/136/img00205cr4.jpg


Thanks, and sorry for my bad english
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Old 14th March 2007, 12:57 PM   #2
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Looks like a quality board. What is the impedance of your headphones? My guess is that it just lacks enough current to drive them.

The opamp is in a socket so it would be easy to try something like OPA2134 to see if it makes any difference, but I doubt it will. If you want more current capability without making any modifications you could try two OPA551 single opamps in a dual adapter.
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Old 14th March 2007, 09:01 PM   #3
PxPert is offline PxPert  Italy
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Unfortunately here in Italy there are no stores of this chips... Can you suggest me any modification to the circuit (resistance,condenser, capacitor) to solve this problem? I'm totally new to this world so please help me!

Thanks!
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Old 15th March 2007, 03:31 AM   #4
Nordic is offline Nordic  South Africa
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If it works otherwise, I would try adding a large value capacitor between the wires from the battery, assuming it runs off 9V you should be able to get a big value 2200uf and more, small size, low voltage one from say, a dead/obsolete PC motherboard...

Looks like a nice opa2228 circuit that, would love to see a schematic, but I guess thats not possible.

Those chips are quite hard to stabalise and are also set up slightly diffirent from other opamps in terms of resitor to ground on input side... so your mileage with other opamps will vary... not saying it won't work.

In my experience opa2228 is one of the best opamps I have heard...provided it is stable...

BTW it is not a cmoy that... anything but. If it works it should be much better.

Let us hear about your progress...
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Old 15th March 2007, 12:48 PM   #5
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Nordic,

I can see from the picture he posted that it is clearly the basic "cmoy" circuit with rail splitter. I can see the 100k input resistor to ground, the gain resistors of 1k/6.04k, and the output resistor of 75R.

I know the headphone amp in the data sheet for the OPA2228 is quite a bit different than the cmoy, but it obviously works with this configuration as well. Maybe that is the problem, I don't know.

There really isn't much you can do to fix the problem PxPert. I suggest you measure the impedance of your headphones so you can get a better idea of what is going on.

If you want, I can mail you an OPA2134 for you to experiment with, though I doubt it will help.
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Old 15th March 2007, 01:28 PM   #6
PxPert is offline PxPert  Italy
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and what about if i change the original capacitor (the one behind the potentiometer) with a bigger one (for example the one you have told)? the original is of 470uf

If you want I can post more photos of the circuit, unfortunately I've not made it so I can't post you the diagrams...
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Old 15th March 2007, 01:31 PM   #7
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I don't think more capacitance will help, but feel free to try. There really isn't any need to post more pics of the circuit. It's fairly obvious what the layout is.
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Old 15th March 2007, 02:16 PM   #8
PxPert is offline PxPert  Italy
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I've tried to add a 1000uf 25v capacitor but with no luck...
So there are no solutions... And if i make an amplifier OPA2228 based from scratch?
Where can I get a simple (and efficent) scheme to build it?
I think with a simple scheme i could make it without problems

Anyway, I've unsoldered most of the upper components and made a photo of the upper and lower side of the circuit, I hope you can make a scheme for this circuit from the photos if you are interested. When I'll come back home I'll post them.

Thanks to theAnonymous & Nordik!
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Old 15th March 2007, 02:39 PM   #9
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PxPert,

Making a completely new circuit for the OPA2228 won't necessarily fix your problem. Like I mentioned before, I believe you have low impedance phones and a low current opamp just isn't going to cut it.

You need to build a circuit capable of more current output.
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Old 15th March 2007, 02:49 PM   #10
Nordic is offline Nordic  South Africa
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If you can figure out which is the feedback resistor, (will go from output to input) try to parallel it with a small cap in the 2 - 12pf range, this should counter any instability...

In my experience that chip does not sound its best in the default cmoy circuit, so a new chip aught to fix the problem... look at ne5532 and opa2312 or opa2134, if you can't find any we can arrange one for you.
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